There's some discussion of this on Mastodon, started by Mythic Beasts, who spotted it yesterday:

https://social.mythic-beasts.com/@beasts/117569497236204746

No, this seems like a big thing that silently happened in the last 24 hours without announcement.

Since roughly November 5, Outlook Hotmail, Live and MSN are rejecting some mail from domains without DMARC records.

This includes low volume domains with passing SPF and DKIM results.

The specific bounce we are seeing is "550 5.7.1 Message rejected: Message lacks SPF/DKIM/DMARC Auth Records".

This is a different outcome from the high volume sender rule (5.7.515) which is part of why this looks like an unannounced extension. I haven't seen anything from Microsoft confirming any change.

The current fix is minimal - just add a DMARC record of something like `v=DMARC1; p=none;` if a domain already has a record though, leave it alone.

I got a notification email about this from mxroute.com (email host) yesterday.

About time.

With how easy spam is now with AI, I welcome the change fully. I’m surprised anyone who reads/posts here wouldn’t have spf and dmarc set up already

Same for us, yes. Error messages started popping up November 5th.

There is another issue that we hit a few months ago which might be related/unrelated. But the total time for Microsoft to lookup your DKIM record and get an answer can't be longer than 500ms or your DKIM fails, their answer is to use faster DNS servers. You'd think 500ms is heaps of time, but if you it include the time to do the NS lookups to see what server to talk to, etc. We were having issue where sending to hotmail/msn from the same account to the same hotmail account would bounce 50% of the time.. It depended on which MS server accepted the incoming SMTP.